The present disclosure relates to a cathode active material, its manufacturing method, and a non-aqueous electrolyte secondary battery. More particularly, the invention relates to a cathode active material containing a composite oxide containing lithium Li and cobalt Co, a manufacturing method of such a cathode active material, and a non-aqueous electrolyte secondary battery using the cathode active material.
In recent years, a demand for a small secondary battery having a high capacitance is increasing associated with the spread of portable apparatuses such as video camera, notebook-sized personal computer, and the like. Among the secondary batteries which are used at present, there is a nickel-cadmium battery using an alkali electrolytic solution. However, its battery voltage is low to be equal to about 1.2V and it is difficult to improve an energy density. Therefore, there has been examined a lithium metal secondary battery using a lithium metal in which a specific gravity is smallest to be equal to 0.534 among those of simple substances of solids, an electric potential is extremely low, and a current capacitance per unit weight is largest among those of metal anode materials.
However, in the secondary battery using the lithium metal for an anode, dendroid lithium (dendrite) is precipitated on the surface of the anode upon charging and grows by charge/discharge cycles. The growth of dendrite causes such a problem that cycle characteristics of the secondary battery deteriorate and, further, the dendrite pierces through a partition film (separator) arranged so that a cathode is not come into contact with the anode and an internal short-circuit is caused, or the like.
For example, as disclosed in JP-A-1987(Showa 62)-90863, a secondary battery in which a carbonaceous material such as cokes or the like is used for the anode and alkali metal ions are doped and dedoped, thereby repeating charging and discharging has been proposed. It has, consequently, been found that a problem of deterioration of the anode due to the repetition of the charging and discharging as mentioned above can be avoided.
As a cathode active material in which the battery voltage of about 4V can be obtained, inorganic compounds such as transition metal oxide containing an alkali metal, transition metal chalcogen, and the like have been known. Among them, a lithium composite oxide such as lithium cobalt acid, lithium nickel acid, or the like is most desirable from viewpoints of a high electric potential, stability, and a long life.
Particularly, the cathode active material mainly containing the lithium cobalt acid is a cathode active material which shows the high electric potential and it is expected to increase the energy density by raising a charge voltage. There is, however, such a problem that if the charge voltage is raised, the cycle characteristics deteriorate. Therefore, in the related art, a method of modifying the cathode active material by mixing a small amount of LiMn1/3Co1/3Ni1/3O2 or the like into the cathode active material and using the material or by coating its surface with another material is used.
In the foregoing technique for modifying the cathode active material by coating the surface of the cathode active material, it is requested to accomplish the high coating performance. Various methods have been proposed to satisfy such a request. For example, it has been confirmed that the method of coating with a metal hydroxide has the excellent coating performance. As such a method, for example, such a technique that the surface of a lithium nickel acid (LiNiO2) particle is coated with cobalt Co and manganese Mn through the hydroxide coating step has been disclosed in JP-A-1997 (Heisei 9)-265985. For example, such a technique that the surface of a lithium manganese composite oxide is coated with a non-manganese metal through the hydroxide coating step has been disclosed in JP-A-1999 (Heisci 11)-71114.
Further, such a technique that tin is used as a cathode material mainly containing a lithium nickel acid has been disclosed in JP-A-2003-123750. Such a technique that tin is used as a cathode material mainly containing a lithium manganese acid has been disclosed in JP-A-2001-185139.
Further, with respect to adsorption of a tungsten acid, for example, a cathode active material having a surface layer containing tungsten and/or molybdenum and lithium is well known in JP-A-2002-75367.
However, if a heating process is executed after the composite oxide particle was coated with the metal hydroxide, there is such a problem that sintering between the particles is liable to progress and the particles are liable to be bound. Thus, if the particles are mixed together with a conductive material or the like when the cathode is manufactured, the bound portion and particles are broken, a crack occurs, the coating layer is peeled off, and a damaged surface of the particle is exposed. In such a damaged surface, an activity is fairly higher than that of the surface formed upon baking and a deterioration reaction of an electrolytic solution and the cathode active material is liable to occur.
It is, therefore, desirable to provide a cathode active material in which chemical stability can be improved by suppressing binding of particles, a manufacturing method of such a cathode active material, and a non-aqueous electrolyte secondary battery which uses such a cathode active material and is excellent in a high capacitance and charge/discharge cycle characteristics.